1. Field of the Invention
This invention relates to heatsinks and methods of making and particularly to a heatsink having an array of pins in which the shape and position of each pin and its orientation with respect to the overall array of pins is selected to optimize dissipation of heat.
2. Prior Art and Information Disclosure
The demand for more efficient heat sinks such as are used for cooling integrated circuit chips and similar electonic devices has increased in proportion to increased power generated per surface area as the industry progresses from one generation of chip development to the next. The fabrication of an "ideal" heat sink of the type that would be useful with electronic components and particularly with integrated circuits poses special problems that are inherent in the objective to maximize area of the heat radiating surface and directing air flow to follow the most efficient pattern. Earlier approaches were characterized as having metal strips parallel to one another and supported with an edge secured to a base plate. Techniques for fabricating this style of heatsink involved extruding metal to form the array of elongated strips. However, the extrusion technique does not lend itself to more complicated constructions that present greater heat radiating surface area. The most recent disclosures are directed toward heatsinks comprising an array of pins wherein each pin has an end secured to a base plate which abuts the surface of the device being cooled.
U.S. Pat. No. 3,524,497 to Chu et al discloses a heat transfer device including a pair of parallel panels in which one panel has an array of cylindrical pins, each having an end secured to one panel surface facing another panel surface to which is secured an array of "tubulator" pins which are basically pins with a rectangular cross section but with the corners of the rectangular cross section curved to provide a more even flow of air along the corridors between pins when the array of tubulator pins intermingles with the array of cylindrical pins. The two opposing base plates are described in this disclosure as being any one of a number of materials (metal or plastic) implying that the ends of the pins are bolted to the base plates. No method for constructing the heatsink is disclosed that would be practical for building the small many-pin heatsinks that are required for today's fast developing integrated circuit industry.
U.S. Pat. No. 5,158,136 to Azar discloses an array of cylindrical pins on a base plate with panels vertically mounted along the outer edge of the array to redirect the flow of air through the array for greater efficiency. The arrangement of the panels in this heatsink is very simple and not necessarily the most efficient arrangement of panels with surfaces for diverting air flow to maximize heat dissipation.
U.S. Pat. No. 5,215,140 to Beane discloses a method for diecasting a heatsink comprising a base plate with an array of cylindrical pins vertically mounted on the base plate. Several characteristics are inherent in the diecasting technique which present limitations on the desired intricacy that can be achieved in fabricating the heatsink. One limitation is the thermal characteristics of the metal mold. (Beane states that the preferred mold described in the method of U.S. Pat. No. 5,215,140 is steel), col. 2 line 37 and the process step of claim 1 involves "solidifying said thermally conductive material is said mold and removing the resultant molded heat sink". The steel mold is a good heat conductor
When a molten aluminum alloy enters the mold, aluminum in contact with the cavity wall solidifies to form a skin and further freezing progresses toward the center of the casting. Freezing is accompanied by shrinkage of the alloy so that voids occur in the casting as a result of variations in rate of freezing throughout sections of the diecast regardless of the degree of evacuation of gases prior to casting. As stated in U.S. Pat. No. 5,215,140 the greatest density that can be achieved in making diecasts such as the pin heatsink is about 90% maximum The diminished density of the part results in a reduced heat conductivity of the diecast thereby decreasing its efficiency as a heatsink.
The rate of cooling is also a factor in determining the crystal size of the cast part. The faster is the rate of cooling then the smaller is the crystal size of the solidified part. This condition is conducive to a more brittle part. Furthermore, small crystal size decreases the heat conductivity of the part so that the heat dissipating capability of the heatsink is diminished. The heat conductivity of the metal mold also places a limitation on the intricacy of the part in terms of the length of part measured from the gate of the mold to the most distal end of the casting versus the cross section of the casting transverse to the length.
The conductive properties of the die cast limit also place a limitation on the length of the part that can be diecast relative to its cross sectional area. In order to extend this limit, aluminum alloys containing a relatively large percent of zinc are preferred for the diecast process. However, concentrations of zinc such are used to manufacture the heatsinks of the type addressed in this disclosure materially reduce the heat conductivity and therefore the efficiency of the diecast heatsink.
Investment casting is a process that has been used for many years to produce intricate castings such as figurines. The technique generally involves forming a wax model of the part to be produced by:
pouring molten wax into a mold such as a flexible mold from which the wax casting can be separated; PA1 pouring investment casting plaster around the wax model to form a plaster casting; PA1 heating the plaster casting to drain and bum out the wax leaving a cavity in the plaster casting; PA1 pouring molten metal into the cavity left in the plaster mold; PA1 breaking the plaster casting away from the metal part.